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Thinking about renting your basement unit or backyard cottage? Washington’s rules changed fast. Here’s what Puget Sound owners need to know before listing.

If you have a basement unit, converted garage, or backyard cottage, you may be sitting on additional rental income. But Washington’s ADU legislation moved quickly between 2023 and 2026, and what was accurate a year ago may no longer apply to your property or city.

What the Law Now Allows

State law now requires every city under Washington’s Growth Management Act to allow at least two ADUs per lot in single-family zones, covering Seattle, Bellevue, Kirkland, Redmond, and Renton. The important caveat is that every city implements this differently. Bellevue, for example, didn’t permit detached ADUs at all until June 2025. If you last looked into this even twelve months ago, the rules for your city may have already changed.

The Mistakes That Cost Owners Money

Renting without proper permits. A finished basement with a separate entrance is not automatically a legal rental unit. In Seattle, all ADUs must be registered under the Rental Registration and Inspection Ordinance before they can be legally occupied. Skipping this step can prevent you from pursuing an eviction, even with a completely legitimate reason.

Misreading the rent cap exemption. Washington’s 2026 rent increase cap is 9.683%. A narrow exemption exists for owner-occupants renting no more than two units on the same property. If you don’t live on-site, your ADU rent increases are capped like any other rental.

Mixing up short-term and long-term rental rules. A standard lease and a short-term platform listing are entirely separate regulatory categories with different licensing requirements. Treating them as the same is one of the most common and expensive errors we see.

Underestimating permitting costs. In Seattle, permit fees typically range from $4,000 to $12,000, with school impact and sewer capacity fees added on top. These rarely appear in contractor estimates but have a real impact on your actual return.

How Davis Property Management Can Help

ADU rules are still evolving across the Puget Sound area. Davis Property Management works with property owners across Seattle, Bellevue, Kirkland, Everett, and the broader region to confirm what’s permitted, what your unit could realistically rent for, and what compliance steps are still outstanding, before you list, not after.

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